Artifacts are in the middle of being migrated from ci_builds to
ci_job_artifacts. The expiration date is currently visible in both of
these tables and the test for whether an expired artifact is present
for a job is complex as it requires checking both the of the tables.

Add two new indexes, one on ci_builds.artifacts_expire_at and one on
ci_job_artifacts.expire_at to enable finding expired artifacts
efficiently.

And until the migration is finished, replace the SQL for finding
expired and non-expired artifacts with a hand-crafted UNION ALL based
query instead of using OR. This overcomes a database optimizer
limitation that prevents it from using these indexes.

When the migration is finished the next version should remove this
query and replace it with a much simpler query on just
ci_job_artifacts. See
https://gitlab.com/gitlab-org/gitlab-ce/issues/42561 for followup.